INSULATION FACTS Facts About Fiber Glass # Insulation and 4 Vapor Retarders Information from NAIMA

In this issue we discuss when and where to use vapor retarders.

vapor retarder is surface. This is usually accom- defined as a material plished by using vapor retarder- or system which ade- faced insulation or unfaced quately retards the insulation plus a separate polyeth- Atransmission of water vapor. For ylene vapor retarder. residential construction, the per- The preceding explanation meance of an adequate vapor makes sense for preventing win- retarder should not exceed tertime condensation, but what 1 perm. In some cases it must be about the summertime when lower. The units of permeance are buildings are cooled instead of grains per hour by square foot per heated? The answer to that ques- inch of mercury pressure differen- tion is that, even though an inte- tial, grain/hr. ft2 in Hg. rior side vapor retarder is on the The main reason to retard the “cold” side in summer, the dew- transmission of water vapor point temperature of hot, humid through building envelopes is to summer air is nearly always prevent it from condensing to lower than the temperature to liquid water within the structure. which we cool buildings. That Components of the building means that an interior side vapor envelope that are outside the retarder will nearly always be insulation, such as roof and wall warmer than the dew-point tem- sheathings, are commonly cooler perature and, therefore, conden- than the dew-point temperature sation will occur only very rarely of the air in the conditioned and will be of very short dura- space. If sufficient moisture vapor tion, causing no problems. contacts those cold surfaces, it This holds true for southern will condense to liquid, just as as well as northern U.S. loca- moisture vapor from humid air tions: The highest dew-point condenses on the inside of win- temperatures in the U.S. occur dows in cold weather. We can in places like Biloxi, MS and avoid condensation on cold sur- Galveston, TX, where dew-point faces by reducing the amount of temperatures sometimes reach moisture vapor that contacts 78°F. Fortunately, this happens them. We do this by installing a rarely; dew-point temperatures warm (interior) side vapor are nearly always 75° or lower. retarder because, unless relative And because winter tempera- humidity is extremely high, mois- tures in Biloxi and Galveston ture will not condense on a warm drop into the 20’s on occasion,

there is some justification for Vapor Retarders and Even when not required to pre- interior side vapor retarders in Blown-in Insulation vent condensation problems, attic those places. vapor retarders may be worth- It is also true that in humid cli- Insulation of any form should not while; their presence may help mates where very little heating is be relied upon to prevent mois- maintain more comfortable required, vapor retarders can be ture movement within an insu- humidity levels. When a vapor placed on the exterior side of lated cavity. Whether blown-in retarder is desired and blown-in insulation without causing prob- fiber glass or cellulose, vapor ceiling insulation is used, a combi- lems in winter. retarders are required unless nation of faced batts/blown-in insu- proper ventilation is provided. lation or a vapor retarder ceiling As with fiber glass batt insulation, paint can be used. Recommendations for materials used for vapor retarders Humid Areas for blown-in insulations must have a perm rating of less than What Determines How A reasonable recommendation 1 perm. In a ceiling where the “Tight” a Vapor Retarder for humid (Gulf Coast) and space above is adequately venti- is Required in Walls? cooler “fringe” areas is: Use lated, a vapor retarder may not either an interior or exterior be required. The exception In general, the colder the climate, side vapor retarder with mod- would be in cases where the cold the tighter the vapor retarder erate permeance. Inset sta- side cannot be ventilated. should be. Also, the more vapor- pled (or Unstapled) kraft If you are reinsulating a home tight the building’s outer skin, the facing, with a permeance of with blown-in insulation, tighter the vapor retarder should about 1 perm, meets this installing a vapor retarder onto be. In milder climates of less than requirement; foil and polyeth- the sidewalls if one has not been 4000 heating degrees days (HDD), ylene do not; their permeance previously installed can be quite inset stapled kraft facing is ade- ratings are much lower. difficult. If the cold side of the quate for most installations. Inset (Omitting the vapor retarder wall cannot be ventilated, it may stapled kraft is also adequate in entirely would work, but be necessary to paint the interior cooler climates in buildings whose would not provide energy-effi- surfaces of exterior walls and outer skins are vapor-permeable, cient cooling. However, if the ceilings with a vapor retarder like wood fiber sheathing and vinyl building structure itself is a forming paint. or aluminum siding. In northern vapor retarder, unfaced insula- areas of 6000+ HDD, face stapled tion would be suitable.) or separate polyethylene vapor If an interior vapor retarder is Do Attics Need Vapor retarders should be considered. used, do not cool the building Retarders? Polyethylene or other tight, contin- below 76°F on the most humid uous vapor retarders should be days. If an exterior vapor retarder Attic vapor retarders are com- used in all but deep south/Gulf is used, do not humidify the monly omitted when blown-in Coast areas when very low perme- building. Keep relative humidity insulations are used. If sufficient ance exterior sheathing/siding below 30% in cold weather. attic ventilation exists, condensa- combinations are used. Check local practice; if it tion problems do not occur in Plywood is commonly used as causes no problems, follow it. most U.S. climates. Sufficient wall sheathing for its structural Also, when specifying conden- attic ventilation is usually defined capabilities. It should be noted sation control strategies for as having a net free ventilating that 1/2˝ exterior grade plywood hot, humid locations, keep in area equal to 1/150 of the attic has a vapor permeance of mind that housewraps are bar- floor area. When an attic vapor approximately 1/2 perm. When it riers to air and liquid water but retarder is used, ventilation is used in areas that experience not to water vapor. Typical requirements are halved; net free more than 4000 HDD, vapor exterior housewraps are not vent area can be 1/300 of the retarders tighter than inset sta- vapor retarders. attic floor area. pled kraft should be considered.

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How are Condensation does not exist. Moving air can duct’s sheathing is not recom- Problems Avoided in carry lots of moisture, but air mended because narrow cuts are Cathedral (Sloped) movement is not necessary for unlikely to significantly increase Ceilings? moisture to escape from buildings. vapor transmission. Facings The best strategy for cathedral should be removed when a Since commonly used asphalt ceilings in cold areas would be to vapor retarder is undesirable. roof shingles have very low vapor use a tight vapor retarder and, if permeance, cathedral ceilings can recessed lights are used, be likened to walls with very low air/vapor-tight fixtures. Addition- Safety Issues permeance exterior skins. As in ally, ventilation openings at eaves walls, the use of very tight, con- and ridges should be provided. Some insulation facings are tinuous vapor retarders can pre- In hot areas, ventilated airspaces intended only for installation vent condensation problems in become more important. When behind ceiling, wall or flooring cathedral ceilings. Problems can ventilated airspaces are provided materials because they are flam- occur, however, if a vapor in milder climate areas, kraft mable. NAIMA recommends that retarder is not continuous. vapor retarders are adequate. an appropriate warning state- Recessed lights are a typical cause ment be printed on all flam- of problems. To prevent loss of Skylight caution: mable facings: conditioned air around recessed Skylights in cathedral ceilings lights, one can cover them with can create unventilated areas This vapor retarder facing is “boxes” constructed of drywall or unless both high and low ventila- flammable and should not be left plywood that are 3˝ from the fix- tion openings are provided. exposed; cover with ceiling, wall or ture. If the vapor retarder is pene- flooring material as soon as possible. trated by recessed lights that are This facing must be installed not air/vapor-tight, some means Should Vapor Retarders in substantial contact with the ceiling, must be provided to allow mois- be Used in Insulated wall or flooring material as required ture to escape. This can be Basements? by building codes. Special care should accomplished with eave and/or be taken when working close to this ridge vents. When both eave and Research indicates that vapor facing with an open flame. ridge vents are provided, a 1/2˝ retarders may not be required or thicker airspace between the for basements, but it is prudent If this insulation is applied over top of the insulation and the roof to keep moisture vapor away existing insulation or between wood sheathing is desirable. This from cold surfaces like basement framing and chimneys, flues or arrangement can also help walls where they are above similar heat sources, the facing should remove heat in hot weather. grade. Therefore, for water-tight be removed. To prevent overheating Note that airspaces without walls in cool climate areas, vapor of recessed light fixtures, do not both eave and ridge vents, will retarders are recommended. insulate on top of, or within 3” of, not add protection against mois- Unfortunately, not all basement such devices. This warning does not ture condensation in sloped ceil- walls are water-tight. While apply to type IC light fixtures. ings; air won’t move through a poured concrete walls are usu- space unless it has a place to exit ally both water and vapor-tight, For exposed applications such as well as a place to enter. block walls are often neither. as unfinished basement walls or Moisture vapor can move For this reason, vapor retarders attic ceilings, insulations with through many materials, including are not recommended for below special strong, reinforced flame fibrous insulation, by diffusion. grade block basement walls resistant facings are available and Therefore, moisture vapor that unless they have been water- must be used. gets around or through a vapor proofed as opposed to the usual Take care to avoid stapling retarder can exit a cathedral “damp-proofed.” Incidentally, into electrical wiring when ceiling rafter bay through a vent when a vapor retarder is not installing faced insulations or sep- opening even when an airspace desired, slashing a faced pro- arate vapor retarders.

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About NAIMA Building Section NAIMA is a trade association of North American manufacturers of fiber glass, rock wool, and slag wool Garage wall insulation products. NAIMA’s role is to promote energy efficiency and environmental preservation through the use of fiber glass, rock wool, and slag wool insulation products and to encourage safe production and use of these insulation products.

For additional information on building insulation contact:

NAIMA 44 Canal Center Plaza, Suite 310 Alexandria, VA 22314 Floor above crawl space Tel: 703/684-0084 Fax: 703/684-0427 E-mail: [email protected] Website: http://www.naima.org

NAIMA BUILDING INSULATION COMMITTEE MEMBERS: CertainTeed Corporation PO Box 860 Valley Forge, PA 19482 800/233-8990

Johns Manville PO Box 5108 Floor of unfinished attic Denver, CO 80217-5108 800/654-3103

Knauf Fiber Glass One Knauf Drive Shelbyville, IN 46176 800/825-4434

Owens Corning One Owens Corning Parkway Toledo, OH 43659 800/GET-PINK

Pitched roof

Roof above suspended ceiling

Printed on Recycled Paper PUB. NO. BI 451 3/99